Process for preparing raw sulfite acid



y 5, 1953 G. HAGLUND 2,637,627

PROCESS FOR PREPARING RAW SULF'ITE ACID Filed Aug. 5, 1947 1521 9123;- G'armr Mal. (0Y0 {penis Patented May 5, 1953 UNETED STATES PATENT 2,637,627 9F F l CE Gustaf Haglund, Storangen, near Stockholm, Sweden Application August 5, 1947', Serial No. 766,244 In Sweden May 3'1, L946 Section 1, Public Law 690, August 8, 1946 Patent expires May 31, 1966 4 Claims.

This invention relates to the preparation of raw sulphite acid of the kind used in the manufacture of sulphite pulp.

The object of the invention is to provide a process and apparatus fer continual production of raw sulphite acid from S02 gases and the desired base material, whereby it is rendered possible considerably to reduce the dimensions of the acid production system and simultaneously to obtain an effective and quick absorption of the S02 gases and more effectively and accurately than hitherto to control and regulate the composition of the acid produced, independent of the working conditions prevailing.

For this purpose, according to the invention, S02 gases, substantially consisting of S02 gases from an SOz-producing furnace sulphur burner gases-are in an absorption phase of the process contacted, in excess, with acid already having the desired content of bisulphite, in an absorption apparatus provided with rotatable stirring devices, wherein said S02 gases and said acid are extremely intimately mixed with each other to build up the strength of the acid with free S02. Hereby, the supply of S02 gases is regulated in such a way that at most 25% of the total S02 content of the gas is absorbed. The remaining S02 gases are then conducted to a bisulphite producing phase of the process, in which they are passed in contact with base material in excess and a solution formed in this phase by the reaction between the S02, said base material and water, which is continually supplied to the plant in a quantity corresponding to the quantity of raw acid to be produced and continually drawn oil from the plant. The conditions of absorption in said bisulphite producing phase of the process is regulated in such a way that the S02 content of the gas is practically completely absorbed, while producing an acid with the desired concentration of bisulphite, which is continually passed into the absorption phase of the process where the enrichment of the acid with free S02 takes place.

Any one of the two above-mentioned phases of the process may consist of one or more stages. So, for instance, in case of producing calcium bisulphite liquor, the bisulphite producing phase of the process may comprise two lime towersa weak acid tower and a strong acid tower-or a lime tower containing limestone in lumps and an absorption apparatus of the above-mentioned. type with rotatable stirring devices containing a suspension of lime in water. In the latter case the S02 gases passing ofi from the absorption or enrichment phase of the process are first conducted into the lime tower containing limestone in the form of lumps in counter-current flow to weak acid passing downwards through the tower, where the main part of the S02 contained in the gas is combined with the lime to produce bisul-ph-ite, and then into the said absorption apparatus in contact with a suspension of lime in water continually supplied to said absorption apparatus, said water being supplied in a quantity substantially corresponding to the quantity of raw acid to be produced and continually drawn ofi from the plant. Ultimately the quantity of water is determined by the quantity of cooking liquor required for the digesters. The poor S02 gas from the lime tower entering said absorption apparatus will beextremely intimately mixed with the lime and water by means of said rotating stirring devices in order to obtain a practicall complete absorption of the S02 of the gas. therein, while producing weal: acid. Said weal; acid. is then passed through the lime tower in countercurrent relationship to the S02 gasesv as described, while producing; raw acid with the. desired concentration of calcium bisulphite, which raw acid is conducted into the first-mentioned phase of the process where the enrichment of the acid with free S02 takes place.

La said embodiment of the process liquor col. lected in the lower part of the lime tower may advantageously be recycled through the lime tower in order to facilitate and promote the dis.- solution of lime and the formation of calcium bisulphite' and. to ensure the production of a raw acid with the desired content of lime.

By way of example the process according to the invention, applied for the" production of cal.- cium bisulphite liquor, isnow described with reference to the accompanymg drawing which shows. one form of a plant according to the invention, comprising two: absorption apparatuses 1 and 3: of the type indicated. above and a lime tower Z, in combination, with which plant the said process may he carried out.

the drawing 1' is an absorption apparatus consisting of a cylindrical container, disposed horizontally, in the centre of which is a rotatable shaft t on. which are mounted radially placed perforated discs or plates 5, which may be replaced by rods, in a suitable number and of a suificient area. for forming a suitable contact surface when rotated by means of the driving mechanism 6-. Upon rotation. said discs or plates pass through the liquid space I in the lower part of the container, through which is conveyed strong acid already having its desired content of calcium bisulphite, supplied from the lime tower 2 through. the pipe 8. A constant. level of liquid is maintained in the container by means of a discharge pipe 9 disposed at a suitable height over the bottom of the container, as shown in the drawing. The gas conduit I 0 serves for supply of S02 gases to the apparatus I and after absorption therein the rest of said S02 gases are passed. through the gas conduit II to the lime tower 2.

By appropriate regulation of the supply of S02 gases in relation to the supply of acid and by a rotation of the 1 the acid with free S02 and deprive the gases of The lime tower 2 is provided with a finer grating I2 and a coarser grating I3 somewhat higher up in the tower serving as supports for limestone in lump form filling the main part of the tower.

By means of a pipe I4, a pump I1, and a pipe I8 with spraying devices I9 arranged at the top of the tower, the lime tower is connected with the absorption apparatus 3. Weak acid produced in said apparatus is by means of the pump I'I transferred to the top of the lime tower and spread over the limestone therein. On its way downwards through the tower 2 the acid trickling down on the limestones meets the S02 gases supplied from the conduit II and passing upwards through the tower. Hereby the main part of the S02 content of the gases is absorbed, and a reaction takes place resulting in the formation of calcium bisulphite (strong acid).

:In the lower part of the tower 2 below the grating I2 is a free space [5, serving as a liquid collecting chamber. Said chamber is by means of a pipe I6 connected with the pump 11 and the pipe I8 and hereby it is possible, by means of said pump, to circulate liquid from the chamber l to the top of the lime tower and to ensure the desired content of lime in the acid. A constant level of liquid is maintained in the chamber I5 bymeans of the before-mentioned overflow pipe 3.

For the discharge of the diluted S02 gases remaining after the absorption in the lime tower there is arranged a gas conduit 20 connecting the upper part of the tower 2 with the absorption apparatus 3.

Said absorption apparatus 3 is of similar type and construction as the absorption apparatus I already described. Consequently, it consists of a cylindrical container having a rotatable shaft ZI on which are mounted perforated discs or plates 22, and which is arranaged to be driven by means of the driving mechanism 23.

The conduit 24 provided with a funnel-shaped opening at the top serves for the supplyof water (from the pipe 25) and of finely divided lime (from the lime pocket 26) to the absorption apparatus 3. A constant level of liquid is maintained in said apparatus by '1 means of the before-mentioned pipe I4, which serves as an overflow pipe. For discharge of the exhaust gases from the apparatus 3 is arranged a conduit 21.

By suitably regulating the proportions of lime introduced into the apparatus 3 as well as the CaO, 1.14% combined S02, 1.14% half-free S02 and about 0.6% free S02, consequently in total about 2.88% S02.

For this purpose, sulphur burner gases containing 12% of S02 are supplied through the conduit I0 to the absorption apparatus I in an amount corresponding to 8.64 kg's. S02 perminute. In the example chosen the acid in the absorption apparatus should be enriched with about 0.4% free S02, it being assumed that the acid supplied from the lime tower 2-300 litres per minutealready contains 0.2% of free S02. This absorption is effected by rotating the discs or contact surfaces 5 with a speed of about 300 to 600 revolutions per minute, and herefor is required 1.2 kgs. S02 per minute corresponding to 14% of the total amount of S02 supplied.

The gas discharged from the apparatus I through the conduit Ii consequently contains 10.32% S02, which means that 7.44 kgs. S02 per minute are supplied to the lime tower 2, this quantity being calculated to be appropriate for producing the correct percentages of calcium bisulphite. The tower 2 also receives 300 litres of weak acid per minute from the absorption apparatus 3 supplied by means of the pump 11 through the pipes l4 and I8 and the spraying devices I9. At the same time the pump II transfers a quantity of acid from the acid collecting chamber I5 at the bottom of the tower 2 through the pipes I6 and It to the top of the tower, and since the pump in the present example has a capacity of about 3 times the capacity of the acid production plant, or 900 litres per minute, 600 litres acid per minute is recirtherein that the S02 content of the acid is in- I creased with 1.87%, which amount is consumed for the formation of calcium bisulphite, with the exception of 0.2%, which is present in the form of free S02. At the same time the base content of the acid is increased with 0.73% CaO.

The gases discharged from the lime tower through the conduit 20 to the absorption ap- 'paratus 3 contain 2.52% S02 and correspond to a quantity of 1.84 kgs.SO2 per minute. In the same time the absorption apparatus 3 receives 300 litres of water per minute and a quantity of finely divided lime, for instance slaked burnt-lime or lime sludge from the causticization ter supplied. By rotating the stirring devices with a speed of about 300 to 600 revolutions per minute the total amount of S02 in the gas is absorbed, while producing a weak acid containing 0.61% S02'and 0.27% CaO in the form of bisulphite.

As will be clear from the foregoing, in, the -="cited example, the whole quantity of water required for producing the desired amount of raw acid is supplied to the final absorption apparatus 3, as well as the quantity of finely divided lime necessary for reacting with the whole amount "of S02 still present in the diluted gases to obtain a complete absorption thereof, while formlllg bisulphite. The weak acid thus formed of the composition already mentioned contains 21% of the total quantity of S02 and 27% of the total content of CaO- of th raw acid to be produced. In the lime tower said weak acid is converted to a strong acid by receiving the remaining r3 70 of CaO and further of the $02 to be contained in the acid. Said strong acid of the compositi0n 1. 00% CaO, 1.14% of CaO,

1.14% half-free S02 and 0.2% free S02 is then in the absorption apparatus 1 strengthened with 0.4% of free S02 corresponding to 14% of the total S02 content of the acid, and from here the raw acid with the desired composition is discharged.

The efficiency of the process described is very high and due hereto the plant for carrying out the process may be of extremely small dimensions and nevertheless have a great capacity.

In case of using only finely divided lime for the production of the acid, the lime tower 2 is replaced by an absorption apparatus of the same type and construction as the absorption apparatus 3 having approximately the same capacity as the tower 2.

For the production of sodium bisulphite liquor it is, likewise, only necessary to use absorption apparatuses with rotatable surfaces of the type described.

I claim:

1. A continuous process for preparing raw sulphite acid which comprises, in an absorption zone, passing a gas containing S02, and obtained substantially from a furnace producing S02, over acid already having the desired content of bisulphite, moving a solid body alternatingly through said acid and gas in said Zone for intimately mixing said acid and gas to increase the free S02 content of said acid, the supply of S02 to said absorption zone and the movement of said solid body being adjusted in such a way that at most 25% of the total S02 content of the gas is absorbed, conducting the remaining S02 containing gases from said absorption zone to a bisulphite producing zone containing a basic material of at least one metal of the group consisting of calcium, and sodium, continually supplying water to said bisulphite producing zone in a quantity corresponding to the quantity of raw acid to be produced, contacting the said S02 containing gases in said bisulphite producing zone with said water and said basic material to produce an acid having the desired content of bisulphite, the conditions of absorption in said bisulphite producing zone being adjusted in such a way that the S02 content of the gas is practically completely absorbed, and passing the acid with the desired content of bisulphite thus produced to said absorption zone.

2. A continuous process of producing raw sulphite acid, which comprises introducing S02 containing gas, consisting substantially of S02 containing gas from an S02 producing furnace, into an absorption zone, containing acid already having the desired content of calcium bisulphite in the absence of lime material, intimately mixing said S02 containing gases with said acid in said absorption zone to increase the free S02 content of said acid, the supply of S02 containing gases to the said absorption zone being so adjusted that at most 25% of the total S02 content of the gas is absorbed, conducting the remaining S02 containing gas from said absorption zone to a bisulphite producing zone comprising a first stage in which the S02 gases are passed through a confined mass of limestone in the form of lumps in counter-current flow to weak acid passing downwards through said mass of limestone, the main part of the S02 contained in the gas being thus combined with the lime to produce bisulphite, and a second stage in which the S02 gases are passed in contact with finely divided lime material and water which is continually supplied to this stage in a quantity substantially corresponding to the desired quantity of raw acid to be produced, mixing the poor S02 gases entering said second stage intimately with the lime material and water, moving a solid body alternately through said lime material and water and said poor S02 gases to obtain a practically complete absorption of the S02 of the gas therein while producing weak acid, passing said weak acid through the limestone mass as described, while producing acid With the desired concentration of calcium bisulphite, and then passing said acid into the absorption zone of the process.

3. The process according to claim 2, wherein liquor produced in first stage of the bisulphite producing zone is recycled through said limestone mass to ensure an efiective dissolution of lime in said stage in order to produce an acid with the desired content of calcium bisulphite.

4. A continuous process for preparing raw sulphite acid liquor containing a predetermined percentage of bisulphite of at least one metal of the group consisting of calcium and sodium, which comprises passing a furnace gas containing sulphur dioxide through a first absorption stage in intimate contact with a liquor comprising an aqueous bisulphite solution obtained in a later stage of the process and which is substantially free from basic substance convertible into bisulphite of said metal, thereby to absorb in said solution at most 25% of the total sulphur dioxide of the gas mixture, contacting the remainder of said gas mixture in a dissolving stage with an excess of basic substance of at least one metal of said group and an aqueous sulphite liquid emanating from a later stage of the process to form an aqueous bisulphite solution, transferring said aqueous bisulphite solution from the dissolving stage to the first absorption stage, passing the remainder of the gas mixture through a second absorption stage in intimate contact with a liquor comprising basic substance of at least one metal of said group and an aqueous liquid thereby to absorb substantially all of the remaining sulphur dioxide of the gas mixture, transferring an aqueous sulphite liquid from said second absorption stage to said dissolving stage, supplying water to said second absorption stage, intimately mixing the gas mixture with the liquor by passing said gas over said liquor and moving a solid body alternatingly through said gas and liquor, and controlling the amount of gas mixture supplied and the amount of sulphur dioxide absorbed in said first absorption stage and the amount of water supplied in said second absorption stage, thereby to obtain in said dissolving stage an aqueous bisulphite solution containing the desired predetermined percentage of bisulphite.

GUSTAF HAGLUND.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,458,310 Richter June 12, 1923 1,576,705 Babcock Mar. 16, 1926 1,616, 03 Richter Feb. 8, 1927 1,747,047 Bradley et a1 Feb. 11, 1930 2,081,112 Statham et al May 18, 1937 FOREIGN PATENTS Number Country Date 56,691 Sweden Mar. 12, 1921 

1. A CONTINUOUS PROCESS FOR PREPARING RAW SULPHITE ACID WHICH COMPRISES, IN AN ABSORPTION ZONE, PASSING A GAS CONTAINING SO2, AND OBTAINED SUBSTANTIALLY FROM A FURNACE PRODUCING SO2, OVER ACID ALREADY HAVING THE DESIRED CONTENT OF BISULPHITE, MOVING A SOLID BODY ALTERNATINGLY THROUGH SAID ACID AND GAS IN SAID ZONE FOR INTIMATELY MIXING SAID ACID AND GAS TO INCREASE THE FREE SO2 CONTENT OF SAID ACID, THE SUPPLY OF SO2 TO SAID ABSORPTION ZONE AND THE MOVEMENT OF SAID SOLID BODY BEING ADJUSTED IN SUCH A WAY THAT AT MOST 25% OF THE TOTAL SO2 CONTENT OF THE GAS IS ABSORBED, CONDUCTING THE REMAINING SO2 CONTAINING GASES FROM SAID ABSORPTION ZONE TO A BISULPHITE PRODUCING ZONEE CONTAINING A BASIC MATERIAL OF AT LEAST ONE METAL OF THE GROUP CONSISTING OF CALCIUM, AND SODIUM, CONTINUALLY SUPPLYING WATER TO SAID BISULPHITE PRODUCING ZONE IN A QUANTITY CORRESPONDING TO THE QUANTITY OF RAW ACID TO BE PRODUCED, CONTACTING THE SAID SO2 CONTAINING GASES IN SAID BISULPHITE PRODUCING ZONE WITH SAID WATER AND SAID BASIC MATERIAL TO PRODUCE AN ACID HAVING THE DESIRED CONTENT OF BISULPHITE, THE CONDITIONS OF ABSORPTION IN SAID BISULPHITE PRODUCING ZONE BEING ADJUSTED IN SUCH A WAY THAT THE SO2 CONTENT OF THE GAS IS PRACTICALLY COMPLETELY ABSORBED, AND PASSING THE ACID WITH THE DESIRED CONTENT OF BISULPHITE THUS PRODUCED TO SAID ABSORPTION ZONE. 